1. Field of the Invention
This invention relates to a method for bonding lignocellulosic material with an aqueous alkaline solution of a phenolic resole resin for the manufacture of structural panels under heat and pressure.
2. Prior Art
Panel products which use phenol-formaldehyde resole resins as binders for lignocellulosic materials are usually manufactured in a hot press which is heated by steam, hot oil, or electricity. In the production of such panels, a phenolic binder, e.g. phenol-formaldehyde resin, is applied to cellulose components such as chips or veneers to form a mat or to otherwise consolidate the components. The consolidated components, also referred to as a mat herein, are then placed between two heated platens and pressed into the composite panels. Conventionally, during pressing, heat is supplied to the mat to plasticize it, thereby making the mat easier to compress and also to cure the thermosetting binder. The time spent in pressing is the major bottleneck in the production of such panels.
The panels can be cured more rapidly by injecting steam into the lignocellulosic mat to heat the resin. This heats the board by injecting steam directly into the board through openings in the press platens. Steam injection press cycle times are generally much shorter than with conventional pressing times since the heat transfer into the board is almost instantaneous.
Liquid phenol-formaldehyde resins do not perform well in steam injection presses, except possibly in a process for making fiberboard. In fiberboard, the wood particles are very small and pliable. It is possible that when phenolic resin is used in steam injection pressed fiberboard the resin saturates the wood and is still available at the wood surface for bonding. When larger wood particles are used the effect is different. Steam injection pressing yields panels that have delaminations in areas that show a loss of the phenolic resin. One explanation for this effect is the penetration of resin into the wood surface in the zone where steam condensate dilutes the resin before there is enough heat to start resin gelation, or thickening. Such penetration diminishes the amount of resin available on the surface of the cellulose components for binding the components and results in poor bonds. This overpenetration is also referred to as "washout" herein. By the time the zone subjected to washout does see enough heat to cure the resin, it is no longer at the wood surface to act as a binder.
Overpenetration of the resin is also a problem with resin coated components of lignocellulosic material when the moisture content is high even when the panel is cured by conventional heated platens without the use of steam injection.
At the present time, the thermoset binders used commercially in steam injection press operations are urea formaldehyde and polymeric isocyanate. Urea resins are not durable and are not used to manufacture exterior panels. Isocyanate is a very efficient and durable binder, but is marked by high cost and health questions. Isocyanate binders also require the use of expensive release agents since they bond wood to metal as well as wood to wood.
Other processes disclosed in the literature use certain gaseous esters, acids, bases, or amines as catalysts when using steam injection of lignocellusosic components coated with phenolic resole resins. However, the use of such catalysts suffer from a number of shortcomings such as: high cost for the catalyst; the need for means to scavenge or recover excess catalyst; many such catalysts are detrimental to health and environment; they can cause unduly rapid cure; have a deleterious effect on the panel; and can have little or no effect on the overpenetration of resin into the cellulosic components.
The use of alkali metal carbonates is well known for thickening phenolic resole resins and accelerating their cure. Such carbonates are applied in solution, such as with water, to the resin before it is applied to the cellulosic components. There are a number of disadvantages to such applications such as increase in viscosity of the resin prior to being applied to the components due to the action of the alkali metal carbonate, non-uniform application of the alkali metal carbonate, and the introduction of additional water to the components.
A prior art document, specifically an English abstract of Soviet Union patent SU 301284-S which has a publication date of Oct. 13, 1969 and is assigned to Leningrad SM, discloses mixing wood chips with a binder, molding the mixture to form a board, treating the surface of the board with a solution of potassium carbonate, and then subjecting the board to hot pressing. It is alleged that such board has improved physico-mechanical properties and that good flameproofing effects are achieved. Although the English abstract from which this information is obtained does not disclose the type of binder used, it is not seen that this reference obviates applicants' invention since, among other reasons, the carbonate is applied as a solution to the surface of the board.
It has now been found that subjecting lignocellulosic components coated with an aqueous alkaline phenolic resole resin solution to gaseous carbon dioxide inhibits overpenetration of resin, increases bond strength and prevents delamination of the finished panel due to application of steam to the components during steam heating and subsequent cure in the preparation of structural panels. The advantages of subjecting the resin coated cellulose components to carbon dioxide is also evident when forming panels on conventional heated platens without steam injection. This is particularly the case when the components have a high moisture content.